Implementation of the function NORMSINV of EXCEL in PHP

NormSInv.php

<?php
/**
 * Returns the inverse of the standard normal cumulative distribution. 
 * The distribution has a mean of zero and a standard deviation of one.
 * Resources: 
 *   http://board.phpbuilder.com/showthread.php?10367349-PHP-NORMSINV
 *   http://www.source-code.biz/snippets/vbasic/9.htm
 * @param  integer $week number of week
 * @return float       sales
 */
function NormSInv($probability) {
  $a1 = -39.6968302866538; 
  $a2 = 220.946098424521;
  $a3 = -275.928510446969;
  $a4 = 138.357751867269;
  $a5 = -30.6647980661472;
  $a6 = 2.50662827745924;

  $b1 = -54.4760987982241;
  $b2 = 161.585836858041;
  $b3 = -155.698979859887;
  $b4 = 66.8013118877197;
  $b5 = -13.2806815528857;
  
  $c1 = -7.78489400243029E-03;
  $c2 = -0.322396458041136;
  $c3 = -2.40075827716184;
  $c4 = -2.54973253934373;
  $c5 = 4.37466414146497;
  $c6 = 2.93816398269878;

  $d1 = 7.78469570904146E-03;
  $d2 = 0.32246712907004;
  $d3 = 2.445134137143;
  $d4 =  3.75440866190742;

  $p_low = 0.02425;
  $p_high = 1 - $p_low;
  $q = 0;
  $r = 0;
  $normSInv = 0;
  if ($probability < 0 ||
    $probability > 1)
  {
    throw new \Exception("normSInv: Argument out of range.");
  } else if ($probability < $p_low) {

    $q = sqrt(-2 * log($probability));
    $normSInv = ((((($c1 * $q + $c2) * $q + $c3) * $q + $c4) * $q + $c5) * $q + $c6) / (((($d1 * $q + $d2) * $q + $d3) * $q + $d4) * $q + 1);

  } else if ($probability <= $p_high) {

    $q = $probability - 0.5;
    $r = $q * $q;
    $normSInv = ((((($a1 * $r + $a2) * $r + $a3) * $r + $a4) * $r + $a5) * $r + $a6) * $q / ((((($b1 * $r + $b2) * $r + $b3) * $r + $b4) * $r + $b5) * $r + 1);

  } else {

    $q = sqrt(-2 * log(1 - $probability));
    $normSInv = -((((($c1 * $q + $c2) * $q + $c3) * $q + $c4) * $q + $c5) * $q + $c6) /(((($d1 * $q + $d2) * $q + $d3) * $q + $d4) * $q + 1);
  
  }

  return $normSInv;
}

thanks for your help :))